Julian Assange's biological father has described his son as a "great dissident" in what he said would be his first and probably only media interview.

John Shipton, who has been attending the Assange extradition hearing in London, spoke to the Spanish newspaper el País and confirmed that his son did not get to know him until his mid-20s.

"I have kept my mouth closed so as not to hinder things," said Shipton, whose name Assange used to register the Wikileaks.org domain name in 2006.

Shipton met Assange's mother, Christine, then aged 17, at an antiques shop on his way to a Vietnam war demonstration – which she joined. Little is known about the relationship, except that it had ended by the time of their son's first birthday – if not earlier. Christine then married theatre director Brett Assange.

Shipton told el País that he first got to know his son after Christine rang his Sydney home in 1996. Assange was 25 at the time. "It was extraordinary," Shipton said. "Certain of his thought processes made it seem like I was staring into a mirror. I could barely believe it. He had the same logic, the same intense curiosity, the same obtuse way of constructing sentences ... that never end."

That meeting coincided with, or came soon after, Assange's 1996 trial for computer hacking – where his lawyer talked of a "really quite tragic" nomadic childhood that saw him attend at least a dozen schools.

His mother became pregnant in her early 20s after she "effectively ran away from home" to Sydney, according to court documents used by Guardian journalists David Leigh and Luke Harding in their book WikiLeaks: Inside Julian Assange's War On Secrecy. The documents state that Shipton "never took up residence or if he did only took up residence for a very short time" and "had no contact with [Assange]".

Assange nevertheless later felt confident enough to use his father's name to register WikiLeaks' internet domain name, re-registering Shipton's nominal address in 2008 as Nairobi in Kenya.

Shipton had worried that his son was a modern Don Quixote. "At that time it seemed as though Julian loved tilting at windmills, but it turned out not to be like that." He warned Assange that he was setting himself tough, idealistic targets. "When someone tells you they want to turn the world upside down, you reply: 'OK, try it. But it's not that easy!'"

Shipton, who is believed to work as a freelance architect in Sydney, said Assange had inherited Christine's fighting spirit. "He is a great dissident, well-prepared for a new era in which direct action is practised via the internet."

He said his son's style of dissidence followed in the tradition of people like Che Guevara, Apollinaire or south American hero Simón Bolívar.

Shipton is convinced Assange is the victim of a conspiracy. "I think all this has been organised," he said, while adding that he did not want to hurt Assange's alleged Swedish victims with his words. "The intelligence agencies got involved in this business from the very start."

Assange's father apparently sees the US government behind the decision by Mastercard and Visa to prevent WikiLeaks accepting donations from their credit cards. "There is no separation between governments and finance," he said.

"There are many intelligent people in the world, but most seem to be wicked, while Julian seems to have the moral courage and ability to carry his vision through. He seems to have an immense desire for justice in the world."